1. Field of the Invention
The present invention relates in general to a thermal type current meter or a thermal type flow meter for detecting a flow velocity or a flow rate of gas or liquid, for example. In particular, the invention relates to a thermal type flow rate detector which is used to detect a quantity of air sucked into an engine for an automobile or the like as an example of application, and which is capable of enhancing the detection accuracy.
2. Description of the Related Art
In general, as for a flow rate detector for measuring a quantity of air sucked into an engine for an automobile, a thermal type one is generally used which can detect a mass flow rate, and which is excellent in cost performance. An engine sucks thereinto air on the basis of the reciprocating motion of a piston and increases or decreases a quantity of sucked air by opening or closing a throttle valve. The pulsation amplitude of air sucked into a 4-cylinder gasoline engine becomes larger as opening of the throttle valve is further increased and in a running state in which the engine becomes the full power, the air sucked there into becomes the pulsation flow entailing the countercurrent. Accordingly, for the accurate detection of a quantity of air sucked into an engine, there is required a flow meter which can detect the countercurrent as well.
As for a thermal type flow meter which can detect a countercurrent, as shown in Japanese Patent Laid-Open No. 10-500490, there has been put into practical use a temperature difference detection type in which a heat generating element is arranged on the surface side of a silicon substrate, and temperature detecting elements are arranged on the upstream side and on the downstream side with respect to the heat generating element, respectively, to detect a flow rate from the difference in temperature between the temperature detecting element on the upstream side and the temperature detecting element on the downstream side when the heat generating element is controlled at a fixed temperature. The temperature detecting element on the upstream side becomes lower in temperature than the temperature detecting element on the downstream side with respect to the flow direction. Thus, since the sign of the temperature difference is reversed between the fair current and the countercurrent, the countercurrent can be detected.
While the heat generating element and the temperature detecting elements are made so as to be of a diaphragm structure of reducing the heat capacity by using the fine patterning of silicon, since the mechanical strength is ensured, the necessary responsibility cannot be obtained and hence the detection accuracy becomes a problem depending on the engine and its running state. In addition, since an output signal depends on the temperature difference between the heat generating element and the temperature of the sucked air, the temperature difference adjustment of the fixed temperature controlling circuit is required every detector. Moreover, since there is required a temperature difference detecting circuit for detecting the temperature difference between the fixed temperature detecting circuit and the temperature detecting elements on the upstream and downstream sides, the circuit is difficult to be miniaturized.
On the other hand, the above-mentioned thermal type flow rate detector carries out the control so that the temperature of the sucked air and the temperature of the heat generating element become fixed, whereas in a thermal type flow rate detector shown in Japanese Patent No. 11-326003, two objects each having a heat generating element and a temperature detecting element are provided, and the measured temperature difference is kept to the value of zero by a control loop, and a signal based on the electric power supplied to the objects for complying with the aforesaid criterion in accordance with which the temperature difference must be zero is used as a flow rate signal.
A conventional thermal type flow rate detector will herein below be described with reference to FIGS. 8 and 9. FIG. 8 is a circuit diagram showing a configuration of a conventional thermal type flow rate detector, and FIG. 9 is a view showing an arrangement of flow rate detecting elements of the conventional thermal type flow rate detector.
In FIG. 8, a bridge circuit is constructed by an upstream side temperature detecting element R2, a downstream side temperature detecting element R3, and fixed resistors R5 and R6. An output signal of the bridge circuit is inputted to a comparator. An output terminal of the comparator is connected to heat generating elements R1 and R4 through switching devices Q1 and Q2.
The upstream side temperature detecting element R2, the downstream side temperature detecting element R3, and the heat generating elements R1 and R4 are temperature sensitive type resistors, made of platinum, nickel or the like, for example, each resistance value of which is changed depending on the temperature, and are formed in the two objects O1 and O2 having a micro-bridge structure as shown in FIG. 9.
In the circuit configuration shown in FIG. 8, the upstream side heat generating element R1 and the downstream side heating generating element R4 are alternately heated so that the upstream side temperature detecting element R2 and the downstream side temperature detecting element R3 are balanced so as to get nearly the same temperature. For example, when the switching device Q1 is in the ON state, the upstream side heat generating element R1 is in the heating state, and the downstream side heat generating element R4 is in the non-heating state. When the temperature of the upstream side temperature detecting element R2 has risen by heating the upstream side heat generating element R1, the voltage appearing at a node between the fixed resistor R6 and the upstream side temperature detecting element R2 is increased, and when it becomes higher than that appearing at a node between the fixed resistor RS and the downstream side temperature detecting element R3, the level of an output signal from the comparator is inverted so that the switching device Q2 is turned ON and the switching device Q1 is turned OFF. Thus, the downstream side heat generating element R4 is heated so that the temperature of the downstream side temperature detecting element R3 is raised, while the temperature of the upstream side temperature detecting element R2 is lowered.
As described above, the electric power is intermittently and alternately supplied to the heat generating elements R1 and R4, whereby the upstream side temperature detecting element R2 and the downstream side temperature detecting element R3 are balanced so as to get nearly the same temperature. The duty ratio of a gate signal of the switching device Q1 becoming a drive signal for the upstream side heat generating element R1 becomes 50% when a flow rate is zero. Then, the duty ratio is further increased as the forward flow rate is larger, while it is reduced as the reverse flow rate is further increased. Thus, the duty ratio is monitored to allow the forward and reverse flow rates to be detected.
In the above-mentioned thermal type flow rate detector, unlike the conventional temperature difference detection system, an output signal hardly depends on the temperature of the heat generating element and becomes a function of only a flow rate. Thus, that detector has the merit that the temperature adjustment for the heat generating element becomes unnecessary. In addition, since the drive signal of the heat generating element feedback-controlled in the detection circuit is used as the flow rate signal, that detector has the advantage in that its responsibility is more speedy than that in the temperature difference detection system. However, if the ON resistance of the switching device for intermittently controlling the heat generating element varies depending on the temperature or the like, its output signal is also changed accordingly. In addition, the flow rate signal is mixed with the noise due to the switching operation, and in particular, when the flow rate sensitivity is low, the SN ratio becomes worse in some cases.
Moreover, since the output signal of the thermal type flow rate detector is the pulse signal, an interface circuit is required in some cases. In other words, in the case where the pulse signal is intended to be inputted to an analog-to-digital converter, a low-pass filter for converting the pulse signal to the analog voltage is required. Then, if an analog voltage having small ripple is intended to be obtained, the cut-off frequency of the low-pass filter needs to be set to a low frequency, and as a result, there is encountered a problem in that the responsibility becomes poor.
In another conventional thermal type flow rate detector shown in FIG. 10, unlike the system for intermittently controlling the heat generating element as described above, heat generating elements R7 and R8 are provided in the outer periphery of a pipe line, and the temperature difference between the heat generating elements is detected using a thermo couple in a (linear) controller to control the heating of the heat generating elements R7 and R8 so that the level of an output signal of the thermo couple becomes zero. In Japanese Patent No. 11-326003, there is no description of the detailed circuit configuration of a (linear) controller. However, considering from other similar embodiments, the (linear) controller concerned is constructed by an analog-to-digital converter for converting the voltage from the thermo couple, a digital-to-analog converter for supplying a heating current to the heat generating elements R7 and R8, and a signal processing portion. While the thermal type flow rate detector shown in FIG. 10, unlike the thermal type flow rate detector shown in FIG. 8, can continuously control the heat generating elements, the circuit configuration is complicated and the miniaturization of the detector becomes difficult since the analog-to-digital converter, the digital-to-analog converter, and the signal processing portion are required.
In addition, in the arrangement of the temperature detecting elements and the heat generating elements as shown in FIG. 9, the upstream side heat generating element R1 is located on the upstream side with respect to the upstream side temperature detecting element R2. Thus, the thermal type flow rate detector shown in FIG. 9 has a problem in that the heat in the upstream side heat generating element R1 is received by the upstream side temperature detecting element R2 through the fluid to reduce the flow rate sensitivity.
As described above, the conventional flow rate detector for carrying out the control so that the temperature difference between the two temperature detecting elements located on the upstream side and on the downstream side respectively becomes zero to detect a quantity of electric power supplied to the heat generating elements at this time suffers the influence of the characteristics of the switching device for intermittently supplying therethrough the electric power to the heat generating elements, and also requires the analog-to-digital converter, the digital-to-analog converter, the signal processing portion and the like. As a result, there is encountered a problem in that the circuit configuration becomes complicated and the detector is difficult to be miniaturized.
Moreover, the temperature detecting elements and the heat generating elements formed in the two objects are arranged in the order of the upstream side heat generating element, the upstream side temperature detecting element, the downstream side temperature detecting element and the downstream side heat generating element from the fair current upstream side. Thus, there is encountered a problem in that the heat of the upstream side heat generating element is received by the upstream side temperature detecting element to reduce the flow rate sensitivity.
In addition, in the prior art, the influence when the fluid temperature is changed is not taken into consideration. As a result, there is encountered a problem in that when the air temperature is largely changed as in the air sucked into the automobile engine, there occurs the error in flow rate detection due to the difference between the temperature dependency of the heat transfer rate in the upstream side heat generating element and the temperature dependency of the heat transfer rate in the downstream side heat generating element.